Specifically for the design. I haven't been on it in a year, so my memory is foggy, but the reason I left was the fact that it seems tailor made to amplify the most paranoid, antisocial perspectives at the expense of good faith and trust.
The following progression of thought comes to mind: many police departments have a mandatory arrest policy for calls in response to alleged domestic violence. Despite the fact that many neighborhoods are, frankly, boring, plenty of nextdoor users are looking for a chance to interact with the app and contribute to "the greater good." It's easy enough to call the police "just in case," but often there are disastrous results[0] that at minimum include the arrest of one of the partners, regardless of guilt. This kind of mindless see-something-say-something excuse marches us daily toward a speech-chilling antisocial breakdown where all that's left to do is clutch our pearls and cower at the thought of interaction. Nextdoor directly provides a channel for the amplification and praise of this modality of thought. While (I hope) most people reject or ignore it, its continued existence is a net negative for neighborhoods and society.
[0] https://www.fox10phoenix.com/news/police-release-video-taken... -- note that Ahwatukee is a wealthy suburb of Phoenix